The pop stars were spotted arriving separately at the Botafumeiro restaurant in Barcelona to celebrate the 26th birthday of their mutual friend, videographer Alfredo Flores.

While Niall arrived in a people carrier with Alfredo, Ariana arrived a short time later, wearing a white top.

The 21-year-old American singer had to fight her way through a crowd of fans as she made her way into the eatery.

Taking to Twitter, she sent her best wishes to the birthday boy: 'Happy birthday to one of my fav human beings on earth @alfredoflores!!!!!! you are an actual angel. a goooooood fkkin egg. i love u, fredo!!'

Low-key: Niall, who last night was seen dining at the same restaurant as Ariana Grande, looked casual in a striped long-sleeved top and distressed denim shorts

Travelling light: The pop star was accompanied by a minder - who carried their trendy black hand luggage

Private: Niall was seen outside the airport's VIP's entrance and the terminal looked deserted as he headed in

Ariana is due to perform at Barcelona's Palau Sant Jordi on Tuesday night.

As well as the meal on Monday, Niall was also pictured partying with Alfredo on the latter's Twitter page on Sunday night.

A picture of Ariana partying the night away with Alfredo was also posted on his Instagram page during the fun, along with the hashtag #itsmybdayinSpain.

Last night: Ariana Grande arrives at the Botafumeiro restaurant in Barcelona, Spain, on Monday night for a friend's birthday dinner

Two weekends ago, Ariana hit out at speculation about her private life in a feminist speech online.

She wrote: 'I am tired of living in a world where women are mostly referred to as a man’s past, present or future PROPERTY/POSSESSION. I do not. Belong. To anyone. But myself.'

She also lashed out at speculation she was dating Niall in an interview with The Sun On Sunday.

The two have been spotted spending time together at his home and a burlesque club while the petite brunette is in London kicking off the next leg of her Honeymoon tour.

'A girl can be friends with someone with a d**k and not hop on it,' she told the British newspaper.

'I'm tired of needing to be linked to a guy. I'm not Big Sean's ex. I'm not Niall's new possible girl. I'm Ariana Grande.'

With his boys: Niall Horan had specially flown to Barcelona to celebrate his friend Alfredo's birthday